Method and apparatus for ranging transmission by mobile station in wireless communication system

ABSTRACT

Disclosed is a ranging transmission method comprising: a mobile station receiving a message including backoff window information from a base station; and the mobile station performing ranging transmission on the basis of the backoff window information, wherein the backoff window information comprises information indicating a size K 0  of a 0-th backoff window, wherein a size K x  of a nx-th (x being a non-negative integer) backoff window is determined according to K x =K 0 /(2 x ) wherein x denotes a ranging retry count.

TECHNICAL FIELD

The present invention relates to wireless communication, and moreparticularly, to a method and apparatus for ranging transmission by amobile station in a wireless communication system.

BACKGROUND ART

Ranging is a procedure by which a mobile station adjusts transmissionparameters (frequency offset, time offset and transmit power) for uplinkcommunication with a base station when the mobile station performsnetwork re-entry and network entry.

Specifically, a ranging procedure is used for the following fourpurposes: initial ranging, handover ranging, periodic ranging andbandwidth request ranging. Initial ranging refers to a process of uplinktime synchronization (i.e., time and frequency synchronization) when auser equipment attempts initial network entry. Handover ranging refersto a process of establishing initial synchronization between a userequipment and a target base station when the user equipment hands overfrom a source base station to the target base station. Periodic rangingis used for a user equipment to periodically update uplinksynchronization. Bandwidth request ranging is used for a user equipmentto request an uplink resource to a base station.

When a mobile station attempts to enter a network, the mobile stationselects a ranging channel, selects a ranging code, and transmits theselected ranging code to a base station through the selected rangingchannel. Upon receipt of the ranging code, the base station transmits,to the mobile station, a message representing that the ranging code hasbeen successfully received.

The number of mobile stations entering a network increases ascommunication technologies develop. When lots of mobile stations in anidle state attempt network entry/re-entry, access collision and accesscongestion occur, resulting in deterioration of communicationperformance. Accordingly, a method for solving this problem is needed.

DISCLOSURE Technical Problem

An object of the present invention is to provide a method and apparatusby which a mobile station communicates with a base station.

Another object of the present invention is to provide a method andapparatus by which a base station communicates with a mobile station.

Another object of the present invention is to provide a mobile stationcommunicating with a base station.

Another object of the present invention is to provide a base stationcommunicating with a mobile station.

Technical Solution

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theobjectives and other advantages of the invention will be realized andattained by the structure particularly pointed out in the writtendescription and claims thereof as well as the appended drawings.

To achieve these and other advantages and in accordance with the purposeof the present invention, as embodied and broadly described, a rangingtransmission method comprising: a mobile station receiving a messageincluding backoff window information from a base station; and the mobilestation performing ranging transmission on the basis of the backoffwindow information, wherein the backoff window information comprisesinformation indicating a size K₀ of a 0-th backoff window, wherein asize K_(x) of a nx-th (x being a non-negative integer) backoff window isdetermined according to K_(x)=K₀/(2^(x)) wherein x denotes a rangingretry count.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a method for receiving rangingtransmission from a mobile station, comprising: a base stationtransmitting a message including backoff window information to themobile station; and the base station performing a ranging procedure withthe mobile station on the basis of the backoff window information,wherein the backoff window information comprises information indicatinga size K₀ of a 0-th backoff window, wherein a size K_(x) of a nx-th (xbeing a non-negative integer) backoff window is determined according toK_(x)=K₀/(2^(x)) wherein x denotes a ranging retry count.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a mobile station configured to performranging transmission to a base station in a wireless communicationsystem, the mobile station comprising: a receiver; a transmitter; and aprocessor configured to control the receiver, wherein the processor isconfigured to control the receiver to receive a message includingbackoff window information from the base station and to perform aranging procedure on the basis of the backoff window information,wherein the backoff window information comprises information indicatinga size K₀ of a 0-th backoff window, wherein the processor is configuredto determine a size K_(x) of a nx-th (x being a non-negative integer)backoff window according to K_(x)=K₀/(2^(x)) wherein x denotes a rangingretry count.

To further achieve these and other advantages and in accordance with thepurpose of the present invention, a base station configured to receiveranging transmission from a mobile station in a wireless communicationsystem, the base station comprising: a receiver; a transmitter; and aprocessor configured to control the receiver and the transmitter,wherein the processor is configured to control the transmitter totransmit a message including backoff window information to the mobilestation and to perform a ranging procedure with the mobile station onthe basis of the backoff window information, wherein the backoff windowinformation comprises information indicating a size K₀ of a 0-th backoffwindow, wherein a size K_(x) of a nx-th (x being a non-negative integer)backoff window is determined according to K_(x)=K₀/(2^(x)) wherein xdenotes a ranging retry count.

Preferably, the ranging transmission method according to claim 1,wherein the message is a paging message.

Preferably, the ranging transmission method according to claim 1,wherein the size K₀ of the 0-th backoff window is greater than 2.

Advantageous Effects

According to the present invention, a mobile station can rapidly andefficiently perform network re-entry while minimizing its influence onother mobile stations in a wireless communication system.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a block diagram of a mobile station and a base stationaccording to an embodiment of the present invention;

FIG. 2 illustrates a contention-based network re-entry procedureperformed by a mobile station in an IEEE 802.16m system which is awireless communication system;

FIG. 3 is a flowchart illustrating a conventional broadband randomaccess backoff method;

FIG. 4 illustrates results of experiments with respect to conventionalrandom access and random access according to a first method of thepresent invention;

FIG. 5 illustrates results of experiments with respect to conventionalrandom access, random access according to the first method of thepresent invention, and random access according to a second method of thepresent invention;

FIG. 6 illustrates results of experiments according to a third methodfor aligning a random access backoff window start point according to thepresent invention; and

FIG. 7 is a flowchart illustrating a random access method according toan embodiment of the present invention.

MODE FOR INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention with reference to the accompanying drawings. Thedetailed description, which will be given below with reference to theaccompanying drawings, is intended to explain exemplary embodiments ofthe present invention, rather than to show the only embodiments that canbe implemented according to the invention. For example, while thefollowing detailed description includes specific details in order toprovide a thorough understanding of the present invention, it will beapparent to those skilled in the art that the present invention may bepracticed without such specific details. For example, the followingdetailed description is given under the assumption that a 3GPP LTEmobile communication system is being used. However, the description isapplicable to any other mobile communication system except for specificfeatures inherent to the 3GPP LTE system.

In some instances, known structures and devices are omitted, or areshown in block diagram form focusing on important features of thestructures and devices, so as not to obscure the concept of the presentinvention. The same reference numbers will be used throughout thisspecification to refer to the same or like parts.

In the following description, a User Equipment (UE) is assumed to referto a mobile or fixed user end device such as a Mobile Station (MS), anAdvanced Mobile Station (AMS), etc. and the term ‘Base Station (BS)’ isassumed to refer to any node of a network end, such as a Node B, anenhanced Node B (eNB or eNode B), an Access Point (AP), etc.,communicating with a UE. The present invention focuses on the IEEE802.16m. However, technical features of the present invention areapplicable to other communication systems such as 3 GPP LTE, LTE-A, etc.

In the present invention, M2M (Machine to Machine) communication meansinformation exchange between mobile stations via a base station orbetween a base station and mobile stations without user intervention. Inview of this, an M2M device means a mobile station capable of supportingM2M communication. An access service network for M2M service is definedas an M2M access service network (ASN) and a network entitycommunicating with M2M devices is called an M2M server. The M2M serverexecutes an M2M application and provides an M2M specific service for oneor more M2M devices. An M2M feature means a feature of the M2Mapplication, and one or more features may be necessary to provide theapplication. An M2M device group represents a group of M2M devices whichshare one or more common features. A device which performs communicationaccording to the M2M scheme, described above, can be referred to asvarious terms such as an M2M device, M2M communication device, machinetype communication (MTC) device, etc. For convenience of explanation, aconventional mobile station is referred to as a human type communication(HTC) mobile station or human-to-human (H2H) device to be distinguishedfrom the M2M device in the following description.

As the number of machine application types increases, the number of M2Mdevices gradually increases in a specific network. Machine applicationtypes under discussion include (1) security, (2) public safety, (3)tracking and tracing, (4) payment, (5) healthcare, (6) remotemaintenance and control, (7) metering, (8) consumer device, (9), pointof sales (POS) and fleet management in a security associated applicationmarket, (10) M2M communication of vending machines, (11) remotemonitoring of machine and equipment, and smart meter for measuringoperating time of construction machine equipment and automaticallymetering heat or electricity consumption, (12) surveillance videocommunication of security cameras, etc. However, the machine applicationtypes are not limited thereto and other various machine applicationtypes are under discussion. As the number of machine application typesincreases, the number of M2M devices can rapidly increase over thenumber of H2H devices.

A lot of M2M devices located in a single base station may cause accesscongestion among the M2M devices and existing devices, that is, H2Hdevices, and access collision between M2M devices. This requires adiscussion about how to efficiently distribute restricted resources tonew M2M devices while minimizing the influence on existing mobilestations (H2H devices). In other words, when a plurality of M2M devicesemploy the procedure of re-entering a network from an idle mode, whichis applied to the existing mobile stations, that is, H2H devices, accesscongestion among the H2H devices and M2M devices occurs due to featuresof the M2M devices, and thus the network re-entry procedure needs to bepartially modified.

A description will be given of an embodiment of the present inventionfor a case in which M2M communication is applied to IEEE 802.16m.However, the present invention is not limited thereto and embodiments ofthe present invention are applicable to other systems such as 3GPP LTE,etc. in the same manner.

FIG. 1 is a block diagram of a mobile station and a base stationaccording to an embodiment of the present invention.

Referring to FIG. 1, a mobile station 100 and a base station 150 mayrespectively include RF units 110 and 160, processors 120 and 170, andoptionally memories 130 and 180. The RF units 110 and 160 mayrespectively include transmitters 111 and 161 and receivers 112 and 162.In the mobile station 100, the transmitter 111 and the receiver 112 maybe configured to transmit/receive signals to/from the base station 150and other mobile stations and the processor 120 may be functionallyconnected with the transmitter 111 and the receiver 112 to control thetransmitter 111 and the receiver 112 to transmit/receive signals to/fromother apparatuses. In addition, the processor 120 may process a signalto be transmitted and then transmit the processed signal to thetransmitter 111, and process a signal received by the receiver 112. Ifrequired, the processor 120 can store information included in anexchanged message in the memory 130. The mobile station 100 having thisconfiguration can perform the following methods according to embodimentsof the present invention. The mobile station 100 may include anadditional configuration according to application type thereof, which isnot illustrated in FIG. 1. If the mobile station 100 is used as a smartmeter, the mobile station 100 can include an additional configurationfor metering power. The mobile station 100 may perform a power meteringoperation under the control of the processor 120 shown in FIG. 1 or aseparately configured processor (not shown).

While FIG. 1 illustrates communication between the mobile station 100and the base station 150, a communication method according to thepresent invention may be performed between mobile stations. In thiscase, apparatuses of the mobile stations can have the same configurationas that of FIG. 1 and perform the following methods according toembodiments of the present invention.

In the base station 150, the transmitter 161 and the receiver 162 may beconfigured to transmit/receive signals to/from other base stations, anM2M server, mobile stations, and the processor 170 may be functionallyconnected to the transmitter 161 and the receiver 162 to control thetransmitter 161 and the receiver 162 to transmit/receive signals to/fromother apparatuses. In addition, the processor 170 may process a signalto be transmitted and then transmit the processed signal to thetransmitter 161, and process a signal received by the receiver 162. Ifrequired, the processor 170 can store information included in anexchanged message in the memory 180. The base station 150 having thisconfiguration can perform the methods according to the embodiments ofthe present invention.

The processors 120 and 170 of the mobile station 100 and the basestation 150 respectively direct (control, adjust, manage, etc.)operations of the mobile station 100 and the base station 150. Theprocessors 120 and 170 may be respectively connected to the memories 130and 180 which store program codes and data. The memories 130 and 180 areconnected to the processors 120 and 170 and store operating systems,applications, and general files.

The processors 120 and 170 may also be called controllers,microcontrollers, microprocessors, or microcomputers. The processors 120and 170 may be configured as hardware, firmware, software, or acombination thereof. When embodiments of the present invention areimplemented using hardware, Application Specific Integrated Circuits(ASICs), Digital Signal Processors (DSPs), Digital Signal ProcessingDevices (DSPDs), Programmable Logic Devices (PLDs), or FieldProgrammable Gate Arrays (FPGAs) which are adapted to implement thepresent invention may be included in the processors 120 and 170.

On the other hand, if the embodiments of the present invention areimplemented using firmware or software, the firmware or software may beconfigured to include a module, a procedure, a function, etc. whichperform functions or operations according to the present invention. Thefirmware or software may be included in the processors 120 and 170, orstored in the memories 130 and 180 and invoked from the memories 120 and170 by the processors 120 and 170.

FIG. 2 illustrates a contention-based network entry/re-entry procedureof a mobile station in an IEEE 802.16m system which is a wirelesscommunication system.

In a ranging procedure for network entry/re-entry of the mobile stationin the IEEE 802.16m system, the mobile station minimizes power in apaging unavailable interval and receives an AAI-PAG-ADV messagetransmitted from a base station in a paging listening interval. TheAAI-PAG-ADV message includes the ID of a paging group to which the basestation transmitting the message belongs, MAC address hash informationindicating mobile stations which need location update or networkentry/re-entry, and an action code describing a procedure which needs tobe performed by each mobile station.

If traffic destined for a mobile station operating in an idle mode ispresent, the base station transmits the AAI-PAG-ADV message to themobile station in the next paging listening interval. Upon receipt ofthe AAI-PAG-ADV message, the mobile station enters a normal mode fromthe idle mode.

A procedure through which a mobile station adjusts transmissionparameters (frequency offset, time offset and transmit power) for uplinkcommunication with a base station when performing network re-entry andnetwork entry from an idle mode is called ranging.

Referring to FIG. 2, the mobile station acquires downlinksynchronization and uplink transmission parameters and selects oneranging channel using random backoff. When the random backoff is used,the mobile station selects one available channel corresponding to abackoff window through a uniform random process. The random backoff usesa binary exponential algorithm in order to calculate the backoff window.Upon selection of the ranging channel, the mobile station selects aranging code through a uniform random process. Then, the mobile stationtransmits the contention based ranging code to the base station throughthe selected ranging channel (S110). The base station transmits anAAI-RNG-ACK message to the mobile station in a broadcast manner whensuccessfully receiving the ranging code (120). The AAI-RNG-ACK messagecorresponds to a response which represents that a ranging code has beensuccessfully received and detected through a ranging channel.Furthermore, the base station masks a CDMA allocation A-MAP IE, whichcorresponds to uplink resource allocation information used for themobile station to transmit an AAI-RNG-REQ message, with a random accessidentifier (RA-ID) and transmits the masked CDMA allocation A-MAP IE(S130). The mobile station transmits the AAI-RNG-REQ message to the basestation through an uplink resource allocated thereto (S140). The basestation transmits, to the mobile station, downlink resource allocationinformation by which an AAI-RNG-RSP message will be transmitted (S150).Here, the downlink resource allocation information may be transmitted tothe mobile station through a CDMA allocation A-MAP IE or a broadcast DKbasic assignment A-MAP IE, which is masked with an RA-ID. Subsequently,the mobile station can receive the AAI-RNG-RSP message through adownlink resource corresponding to the downlink resource allocationinformation (S160).

FIG. 3 illustrates a procedure by which a mobile station increases abackoff window size and performs contention based network re-entry in anIEEE 802.16m system which is a wireless communication system.

MAC layer of IEEE 802.16 defines a ranging channel. Sub-channelsconstituting the ranging channel are described in a UL-MAP message.Mobile stations are allowed to collide with each other on the rangingchannel. In an IEEE 802.16 ranging subsystem, a plurality of mobilestations perform ranging using a code division multiple access (CDMA)random access protocol for simultaneous access. When a message to betransmitted from a mobile station to a base station is generated, a codeis selected from a set of pseudo-noise (PN) codes, and the message isspread using the selected PN code and transmitted through CDMA.

In general, slotted ALOHA protocol is used as a random access protocolof a mobile station. According to the slotted ALOHA protocol, a mobilestation performs slot-based immediate access in order to access a basestation. Accordingly, data is successfully transmitted when a singlemobile station attempts to access a base station through one slotwhereas data transmission fails due to collision when two or more mobilestations attempt to access the base station through the same slot. Whendata transmission fails, a mobile station attempts to access the basestation after waiting for a predetermined time. If mobile stations,which have failed in data transmission, attempt to access the basestation after waiting for the same time, data transmission fails againdue to collision therebetween. To avoid this continuous collision, abinary exponential backoff (referred to as ‘BEB’ hereinafter) algorithm,which is a mobile station backoff algorithm, is employed. The BEBalgorithm determines a backoff window for retransmission on the basis ofthe number of collisions of each mobile station. The backoff window (orcontention window) in the BEB algorithm includes a minimum contentionwindow and a maximum contention window.

The exponential backoff algorithm is a widely used collision resolutionalgorithm. The exponential backoff algorithm attempts to access arandomly selected channel within a contention window for retransmission.The size of a contention window exponentially increases according to thenumber of collisions. According to BEB, a mobile station, which sensescollision, increases the size of a contention window with an exponent of2. The mobile station randomly determines a ranging transmission timewithin the contention window with the exponentially increasing size. Forexample, a contention window of a mobile station which is subjected tocollision during the first transmission trial has a size of 8, themobile station determines a ranging transmission time within 8 framesstarting from the current channel for the second transmission trial. If5 is selected, the mobile station attempts ranging transmission at thefifth channel. Here, 5 corresponds to a backoff value or selectedranging opportunity. When collision occurs again, a collision countbecomes 2 and the size of the backoff window for retransmission, whichis the contention window, becomes 16. Then, the third transmission trialis randomly determined within 16 frames starting from the collisionoccurrence time. That is, collision continuously occurs, the systemrecognizes that load has increased and reduces the system load due toretransmission at the current time by increasing the size of the backoffwindow corresponding to the contention window to thereby resolve thecollision. However, a mobile station subjected to a plurality ofcollisions continuously exponentially increases the contention windowsize even when other mobile stations do not use a radio band occupied bythe mobile station, resulting in remarkable transmission latency.

Referring to FIG. 3, initial data to be transmitted from a mobilestation 200 to a base station 250 is immediately delivered from themobile station 200 to the base station 250 (211). In the above-mentionedrandom access scheme, messages can be transmitted using the same PN codethrough the same slot. In this case, messages transmitted from aplurality of mobile stations collide with each other. Mobile stationswhich have transmitted messages operate timers (213). The base station250 transmits a ranging response message to the mobile stations uponreceipt of the messages. When the mobile stations do not receive theranging response message within a predetermined time, the mobilestations consider that the transmitted messages are subjected tocollision and execute the BEB algorithm. Since collision occurs once, anarbitrary integer is selected in a backoff period (215) of CW_(min)X2⁰.This integer becomes backoff time (215), that is, ranging opportunity.The mobile stations retransmit messages after waiting for the backofftime (217). Here, CW (Competition Window) means a contention window sizeand may refer to a backoff window size. CW_(min) denotes an initialbackoff window size and CW_(max) denotes an available maximum backoffwindow size. Retransmission is performed in the same manner as theinitial transmission. The mobile stations operate the timers (219), anddetermine backoff time through the BEB algorithm when they do notreceive the ranging response message within the predetermined time.Because collision occurs twice, an arbitrary integer is selected in abackoff period (221) of CW_(min)X21. This integer becomes backoff time(221). The mobile stations retransmit messages after waiting for thebackoff time (223).

CW=CW_(min)×2^(x)  [Math Figure 1]

The backoff period increases as the number of collisions increases(225). This backoff period cannot exceed CW_(max), and thus the backoffperiod corresponds to min (CW_(min)*2^(k−1),CW_(max)). Here, k denotesthe number of collisions. The IEEE 802.16 system limits the maximumnumber of collisions to 15. In this case, CW_(max) can have a value ofCW_(min)*2¹⁴ Data is successfully transmitted when the ranging responsemessage is received from the base station (227) within 15 transmissionattempts and data is abandoned when the sixteenth collision occurs. Whenthe number of mobile stations which share the same channel in a wirelessnetwork increases, collision between mobile stations is inevitablebecause the mobile stations may simultaneously transmit messages becausethey do not know when other mobile stations start to transmit messages.To reduce this inevitable collision, the random access protocol uses theBEB algorithm as described above. The BEB algorithm sets a backoffperiod according to the number of collisions of each mobile station anddetermines a backoff time in the set backoff period. Each mobile stationimmediately transmits a message after waiting for the determined backofftime. When collision occurs again, the aforementioned procedure isrepeated. The backoff period increases as the number of collisionsincreases.

The BEB algorithm distributes mobile stations efficiently when thenumber of mobile stations is small compared to radio channel capacity.However, a single mobile station rarely attempts to access a radiochannel when the number of mobile stations increases, and all radioresources may not be used if the number of mobile stations furtherincreases. That is, the BEB algorithm is not suitable when the number ofmobile stations which access one radio channel increases. The currenttrend is an increase in using M2M devices, causing an increase in thenumber of mobile stations which access one radio channel.

The majority of conventional communication has been human-to-human (H2H)communication using user devices via a base station. However, thedevelopment of communication technology enables M2M communication. M2Mcommunication means communication between electronic devices. While M2Mcommunication means wired or wireless communication between electronicdevices in a broad sense, it generally represents wireless communicationbetween electronic devices.

Although M2M communication was recognized as remote control ortelematics and the market therefor was very limited in the earlynineties when M2M communication was introduced, M2M communication hasgrown rapidly in past years to create an attention-drawing market abroadas well as domestically. Furthermore, as application types of M2Mdevices are diversified, lots of M2M devices will be present in the samebase station. When a lot of M2M devices in an idle state attempt networkre-entry, numerous access collisions and access congestion may occur andcommunication performance may be deteriorated. However, there has yet tobe proposed a network re-entry procedure of M2M devices in an idlestate, which have features different from conventional mobile stations(H2H devices).

When the number of mobile stations increases, the number of mobilestations which attempt to access a system also increases. However,little mobile station successfully accesses the system using the BEBalgorithm, as described above. In other words, the system becomesunstable. The BEB algorithm cannot rapidly recover the unstable system.Furthermore, according to the BEB algorithm, a mobile station whichperforms initial ranging and a mobile station subjected to severalcollisions compete against each other in the same slot.

While the mobile station subjected to several collisions needs to accessthe system prior to the mobile station which performs initial ranging,the mobile station subjected to several collisions suffers long latencydue to a backoff period rapidly increased due to a binary exponentincrease. This is called ‘fairness problem’.

The number of collisions of a mobile station which has successfullyaccessed the system is initialized to ‘0’. When the number of mobilestations which attempt to access the system increases, a latentcollision problem wherein collision probability further increases whensuccessful mobile stations re-access the system is encountered.

Therefore, the conventional BEB algorithm has problems that a radiochannel state cannot be considered when a backoff time is determined,efficiency is rapidly deteriorated when the number of mobile stationsincreases, and a system unstable state cannot be rapidly recovered, inaddition to fairness problem and latent collision problem. To solvethese problems, the present invention proposes a method of determining awindow size for retransmission when initial ranging transmission fails.The method of setting an appropriate window size, proposed by thepresent invention, will now be described. According to the method of thepresent, an initial backoff window size can be determined inconsideration of the number of mobile stations which attempt to access abase station and a mobile station group rather than being determined bya mobile station without the control of the base station, and the basestation can transmit information required to determine the backoffwindow to mobile stations. Accordingly, each mobile station can performrandom access within the initial backoff window size, signaled by thebase station, according to the mobile station group to which each mobilestation belongs. The base station can transmit an initial backoff windowsize parameter to the mobile stations on the basis of the number ofmobile stations which successfully access the base station in one frameand the number of access attempts.

A first method proposed by the present invention maintains a backoffwindow size. According to the first method, a base station provides abackoff window size to be used by mobile station(s) to the mobilestation(s).

Since ranging access trials are distributed in such a manner that mostranging access trials are concentrated on earlier opportunity, it isdifficult to ensure successfulness of ranging access. In other words, itis necessary to distribute access loads within a specific window sizeconsidering that the access loads are concentrated into preceding frameswithin the backoff window. If one random access is allowed in one frameto enable ranging, almost all mobile stations fail in accessing a basestation in the first random access procedure.

To solve this, the base station can download information about thespecific window size to a mobile station such that the mobile stationcan randomly select a ranging channel within the specific window size.The successfulness of ranging trail of an M2M device depends on how thespecific window size is set for every ranging trail.

FIG. 4 illustrates results of experiments with respect to conventionalrandom access and random access according to the first method of thepresent invention. The first method according to the present inventionis applicable to a case in which communication load is heavy due to aconsiderably large number of mobile stations. As M2M device applicationtypes such as a smart meter are diversified, the number of M2Mcommunication devices can be remarkably increased compared to the numberof general communication devices. If lots of M2M devices communicatewith a base station through a conventional one-to-one communicationscheme, network overload due to signaling between the M2M devices andthe base station is expected. The M2M devices have characteristics thatthey are in a long sleep mode and attempt to access a network totransmit a small amount of data within a short time. Accordingly, whenmore than 30,000 M2M devices, such as smart meters, in a group aresimultaneously operated, collision occurs in the conventional BEBscheme. Therefore, the present invention proposes a method of increasingthe initial backoff window size for random access in order to reducecollision.

For graphs of FIG. 4, it is assumed that one mobile station groupincludes 200 devices, the horizontal axis represents opportunitycorresponding to one frame, and ranging is successfully performed withone opportunity per frame. The vertical axis of the graphs representsthe number of ranging retrials.

FIG. 4( a) illustrates a result of an experiment with respect to randomaccess performed under the condition that the initial backoff windowsize is set to 2 and one mobile station group includes 200 devices.

When the initial backoff window size is set to 2, it can be seen fromFIG. 4( a), that distribution is not made and opportunities areconcentrated in preceding frames. That is, when the backoff time israndomly determined, a short backoff time is set in many cases.Accordingly, the number of initial ranging retrials reachesapproximately 40 and ranging trials of most mobile stations fail due tocollision. FIG. 4( b) illustrates a result of an experiment with respectto random access performed under the condition that both the initialbackoff window size and a backoff window size for retransmission are setto 60 and one mobile station group includes 200 devices. In this case,distribution is achieved compared to the case of FIG. 4( a) and thenumber of ranging trials is remarkably reduced. That is, it is possibleto reduce the number of collisions occurring during the first rangingtrial by increasing the initial backoff window size suitable for mobilestations operating in the conventional one-to-one communication schemefrom 2 in consideration of a large number of M2M devices. Accordingly,the probability of success of the initial random access trial can beincreased and the number of random access retrials following the initialrandom access trial can be reduced, to thereby decrease latency.Therefore, the base station can set an appropriate initial backoffwindow size by estimating the number of M2M mobile stations which accessthe base station to reduce the number of random access retrials andachieve uniform distribution. It can be seen from FIG. 4 that the numberof ranging retrials due to collisions is remarkably reduced, the numberof collisions is decreased, and the rate of success increases by 50%when the initial backoff window size k is increased from 2 to 60.

When a mobile station (or mobile stations which belong to an M2M group)attempts network entry/re-entry, the mobile station selects a specificranging opportunity in a backoff window size randomly (or according to aspecific rule) in order to avoid access congestion. At this time, themobile station can be classified as one group and predetermined groupscan simultaneously perform network entry. The base station can adjust awindow size for retransmission on the basis of the number of mobilestations which attempt access and the number of mobile station deviceswhich belong to one mobile station group and transmit an initial backoffwindow size parameter to the mobile station. The mobile station canperform random access using the initial backoff window size parameterreceived from the base station.

Here, when the mobile station cannot receive a success acknowledgementsignal within a predetermined time although it has initially transmitteda ranging signal, the mobile station can attempt access byretransmitting the ranging signal.

The BEB algorithm which randomly selects an opportunity in a window sizeincreased by scaling the initial backoff window size by 2̂x (x denotes aparameter regarding the number of retrials) has been described above.

The second method of the present invention attempts retransmission whilereducing the backoff window size signaled by the base station,distinguished from the conventional BEB algorithm. According to thesecond method of the present invention, the base station transmitsinformation representing an initial backoff window size of greater than2 to a mobile station as in the first method. In addition, the basestation determines one initial value (referred to as K) and transmitsthis value to the mobile station. Distinguished from the first method inwhich the initial backoff window size equals the backoff window size forretransmission, the second method of the present invention reduces thebackoff window size whenever the number of retrials increases. Forexample, the mobile station can calculate a window size P in which anopportunity will be selected for retrial according to the following mathfigure.

P=k/(B ^(x))  [Math Figure 2]

Here, B is an integer greater than 1 and x denotes the rangingretransmission count of the mobile station. For example, when B is 2,P=k/(2¹) for the first retransmission and P=k/(2²) for the secondretransmission. When the opportunity selected by the mobile station forretransmission is Q, Q can be represented as follows.

Q=n+random_selection_of_opportunity_between(0˜P)  [Math Figure 3]

Here, n denotes opportunity (backoff time or backoff value) in the eventof previous transmission.

It is desirable that the window size in which the opportunity will beselected for retransmission has a value equal to or greater than 2.

When the base station does not set the window size to an exponentialfactor of 2, P can be set to one of the following three.

-   -   P=floor (K/(2̂x)); The window size is aligned such that it does        not exceed a window size boundary set with respect to        retransmission.    -   P=ceil (K/(2̂x))    -   P=f(K/(2̂x)) where f(y) is a function which outputs an        exponential factor of maximum 2, which does not exceed y. For        example, when K is 300, P corresponds to 2̂7=128 when x=1 and to        2̂6=64 when x=2.

FIG. 5 illustrates results of experiments with respect to conventionalrandom access, random access according to the first method of thepresent invention, and random access according to the second method ofthe present invention.

FIG. 5( a) and FIG. 5( b) correspond to FIG. 4( a) and FIG. 4( b), andFIG. 5( c) illustrates a result of an experiment to which the secondmethod of the present invention is applied under the condition that boththe initial backoff window size and backoff window size forretransmission in FIG. 5( a) and FIG. 5( b) are set to 60. FIG. 5( c)illustrates a result of an experiment with respect to random accessperformed under the condition that the initial backoff window size isset to 256 and the backoff window size for retransmission is graduallyreduced at the rate of 2^(x) as an inverse exponential backoff windowsize. In the second method of the present invention, the backoff windowsize can be set such that x in math figure 1 is represented as anon-negative integer. When the inverse exponential backoff window sizeis gradually reduced at the rate of 2^(x), distinguished from the casein which the backoff window size is exponentially increased (FIG. 5(a)), significant gain increase can be achieved as illustrated in FIG. 5(c). Referring to FIG. 5( c), a retransmission trial count is maintainedas 1 until the number of frames reaches 200, and thus ranging can besuccessfully performed through one trial. Since random access successcan be achieved within three random access retrials, even though theinitial backoff window size is set to 256, the system can be implementedwithin a window of 512 because the second backoff window size is 128 andthe third backoff window size is 64. This reduces latency. That is, whenthe initial backoff window size is set to 256 and the backoff windowsize for retransmission is reduced, a significant gain increase can beachieved.

Therefore, as shown in the table of FIG. 5, the second method accordingto the present invention can obtain a success count of 102, as shown inFIG. 5( c), for the same ranging trials, 124.

The second method according to the present invention increases theinitial backoff window size based on the distribution characteristic inwhich access loads are concentrated in preceding frames and reduces thebackoff window size for retransmission according to the characteristicin which the number of access loads distributed in later framesdecreases. Furthermore, complexity of implementation can be reduced byinversely applying the conventional method, used to transmit informationon the initial backoff window size, to retransmission. That is, theconventional method can be employed while using an inverse exponentialbackoff window size instead of the exponential backoff window size, tofacilitate implementation. The second method of the present inventioninversely reduces the backoff window size by an exponent of B (B being apositive integer greater than 1) and sets a large initial backoff windowsize so as to reduce the number of frames requested for ranging, therebydecreasing a random access retrial count and latency.

The first and second methods of the present invention set the initialbackoff window size according to the BEB algorithm, to a specific windowvalue, preferably, a value greater than 2. For the first ranging trial,most ranging procedures are subjected to collision when the initialbackoff window size is 2 as in the conventional BEB scheme. Accordingly,in the first and second methods proposed by the present invention, thebase station detects the number of mobile stations capable of accessingthe base station, adjusts the initial backoff window size, includes theadjusted initial backoff window size in a paging message, and transmitsthe paging method to the mobile stations. The number of ranging trialsperformed when a ranging acknowledgement message is not received can bereduced by increasing the initial backoff window size compared to theconventional initial backoff window size. That is, a mobile stationattempts ranging in an appropriate initial backoff window size toachieve successful access within 3 ranging trials in the base of FIG. 5(c).

Tables 1 to 9 show experimental examples for increasing success ratewith a small delay by varying a paging cycle and an access rate when thewindow size is set. The access rate is 40/s in Tables 1, 2 and 3, andthe paging cycle is in Table 1, 2.5 s in Table 2, and 5 s in Table 3.

TABLE 1 Terms A B Targeting success rate Over 99% Over 99% Initialbackoff window size 4 64 Average delay(required 0.53 s(102 frames) 0.3s(60 frames) frames)

TABLE 2 Terms A B Targeting success rate Over 99% Over 99% Initialbackoff window size 8 256 Average delay(required 1.23 s(246 frames) 0.98s(196 frames) frames)

TABLE 3 Terms A B Targeting success rate Over 99% Over 99% Initialbackoff window size 16 512 Average delay(required 2.55 s(510 frames)1.96 s(392 frames) frames)

The access rate is 60/s in Tables 4, 5 and 6, and the paging cycle is inTable 4, 2.5 s in Table 5, and 5 s in Table 6.

TABLE 4 Terms A B Targeting success rate Over 98% Over 98% Initialbackoff window size 16 128 Average delay(required 1.22 s(244 frames)0.58 s(116 frames) frames) *0.99 s w94%

TABLE 5 Terms A B Targeting success rate Over 98% Over 98% Initialbackoff window size 128 256 Average delay(required 4.51 s(902 frames)1.17 s(234 frames) frames) *2.36 s w91%

TABLE 6 Terms A B Targeting success rate Over 98% Over 98% Initialbackoff window size 128 512 Average delay(required 7.64 s(1528 frames)2.33 s(466 frames) frames) *4.74 s w91%

The access rate is 80/s in Tables 7, 8 and 9, and the paging cycle is inTable 7, 2.5 s in Table 8, and 5 s in Table 9.

TABLE 7 Terms A B Targeting success rate Over 90% Over 90% Initialbackoff window size 128 256 Average delay(required 4.16 s(832 frames)1.55 s(310 frames) frames)

TABLE 8 Terms A B Targeting success rate Over 90% Over 90% Initialbackoff window size 128 512 Average delay(required 9.74 s(1948 frames)2.86 s(572 frames) frames)

TABLE 9 Terms A B Targeting success rate Over 90% Over 90% Initialbackoff window size 128 1024 Average delay(required 15.3 s(3060 frames)5.57 s(1114 frames) frames)

As shown in Tables 1 to 9, the present invention can achieve significanttechnical effects compared to the conventional BEB scheme.

Referring to Tables 1 to 9, the access rate was set to various values of40/s, 60/s, and 80/s and the paging cycle was set to various values of 1s, 2.5 s and 5 s and a targeting success rate according to the initialbackoff window size was analyzed. The above-mentioned BEB algorithm,corresponds to A and the method of reducing the backoff window size andadjusting the backoff window start time corresponds to B. As shown inTables 1 to 9, in the method B of reducing the backoff window size, apaging trial overlapping the next paging interval does not occur in thecase of access rates of 40/s and 60/s. An overlapping paging trial iscalled spill over. In order to prevent overlapped paging from occurring,the initial backoff window size is set to a large value and a paginginterval start point for ranging retransmission is signaled with thebackoff window size gradually decreased so as to reduce collision withmobile stations according to the next ranging retransmission. Contentionincreases as the access rate increases. Accordingly, the base stationneeds to set an appropriate backoff window size based on the number ofmobile station devices. When random access is successfully made within 2or 3 random access trials in the initial backoff window size set by thebase station, latency according to random access retrial can bedecreased. Referring to Tables 1 to 9, scheme B can reduce averagedelay, that is, the number of required frames, as compared to scheme Ato accomplish the same success rate at the same access rate and the samepaging cycle. Even when the initial backoff window size is larger in thecase of B than in the case of A, the number of requested frames afterthe initial backoff window size is set is reduced, and thus scheme B canreduce a random access retrial count and latency.

However, at 60/s, scheme A increases the number of requested frames andgenerates spill-over due to overlapped paging. A description will begiven of a third method for preventing spill-over according to thepresent invention.

FIG. 6 illustrates results of an experiment with respect to the thirdmethod of the present invention according to random access backoffwindow start time adjustment.

*109 To reduce spill-over which causes an overlapped frame as shown inTables 1 to 9, the present invention proposes a method for aligning thebackoff window start time.

The third method of the present invention can optimize the second methodproposed by the present invention. The third method determines a startpoint of an i-th backoff window region in consideration of an end pointof an (i−1)th backoff window region.

According to the aforementioned first and second methods, whentransmission fails at a specific opportunity within the backoff windowsize, an opportunity within the next window size is selected on thebasis of the transmission failure time. In the case that the windowstart time for retransmission is determined in this manner in the secondmethod of the present invention and transmission fails at a precedingopportunity of the backoff window, when a mobile station selects anopportunity within a reduced window size for the next transmission, theprobability that the transmission overlaps with the first transmissionof another mobile station at an opportunity at the rear part of thebackoff window is high. For example, when the initial backoff windowsize is set to 256, a random access time can be randomly selected withinthe window of 256 during a ranging procedure. If random access fails dueto collision when the random access is attempted at the 64^(th) framesof 256 frames, which corresponds to the front part of the backoffwindow, random access is retried. The second window backoff size for thefollowing random access becomes 156/(2̂1), that is, 128. Accordingly,when retransmission random access is performed within the window with128 frames after 64 frames in which the first random access isattempted, this random access retrial may overlap with the initialrandom access trial performed by another mobile station within theinitial backoff window size of 256 (for example, random access trialperformed at 160^(th) frame) to further increase collision probability.Therefore, the present invention proposes a method of determining abackoff window size start point in order to prevent window regions inwhich the second random access and the first random access are made fromoverlapping each other. In the above-mentioned example, when randomaccess at the 64^(th) frame fails due to collision, the backoff windowsizes tart point is set to the 256^(th) frame such that the next randomaccess is attempted at a time after the 256 frames.

Therefore, the third method applies a time gap corresponding to aspecific delay for the next retransmission in order to avoidoverlapping. When the opportunity selected by a mobile station forretransmission is Q, Q is represented by the following math figure.

Q=n+random_selection_of_opportunity_between(0˜P)+time_gap,time_gap=(window_size−n_of_previous_transmission)  [MathFigure 4]

In math figure 4, n denotes the opportunity of previous transmission.Here, the time gap can be set as math figure 4 considering a mobilestation which selects the last opportunity of the window for the firsttransmission.

In other words, the opportunity selected by the mobile station forretransmission, Q, can be represented by the sum of the opportunity (avalue selected from 0 to P) randomly selected in the window size P andthe window size of previous transmission,

$\sum\limits_{k = 0}^{k = {i - 1}}\; {P_{k}.}$

The third method proposed by the present invention can set

$\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}$

as the backoff window start point such that the mobile station canattempt ranging at the backoff window start point. When the window sizeof previous transmission is added, the random access trial does notcollide with a random access trial in the window size of previoustransmission, which is performed by another mobile station.

P can be set using the aforementioned second method, and Q is set pertransmission.

When the time gap is set as described above, Q_(i) of i-thretransmission can be represented as follows (sum of Qo fall previoustransmissions).

$\begin{matrix}{Q_{i} = {{\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}} + {{random\_ selection}{\_ under}{\_ P}_{i}{\_ window}{\_ size}}}} & \left\lbrack {{Math}\mspace{14mu} {Figure}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Here, P₀ denotes the initial backoff window size K, and P_(i) denotesthe i-th backoff window size.

$\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}$

an be regarded as the backoff window start point of the i-thretransmission. By setting the backoff window start point in thismanner, it is possible to prevent collision due to ranging retrialaccording to overlap with the first transmission of a different mobilestation at a later opportunity. The base station can set the initialbackoff window size and signal it to the mobile station. The mobilestation can calculate a backoff window size for retransmission and abackoff window start point using the initial backoff window size andperform a random access procedure.

Here, a specific processing delay factor of the system can beconsidered. For example, a time ranging from when a ranging code istransmitted to when a ranging acknowledgment (AAI-RNG-ACK, or RNG-SCK ina 802.16e system) message is received is T₃₁(T₃ in 802.16e). The presentinvention can determine the backoff window start time in considerationof T₃₁ as follows.

$\begin{matrix}{{{backoff\_ window}{\_ start}{\_ point}(i)} = {\max \left( {{\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}},{Q_{i - 1} + T_{31}}} \right)}} & \left\lbrack {{Math}\mspace{14mu} {Figure}\mspace{14mu} 6} \right\rbrack\end{matrix}$

Since random access exceeding the backoff window size for the i-thretransmission may be performed due to a delay according to the timetaken to receive the ranging acknowledgement message, collision due tomessage reception time delay can be avoided by setting the backoffwindow start point to a maximum value between the sum of backoff windowsizes of 0-th to (i−1)th retransmissions,

${\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}},$

and the sum of the (i−1)th ranging opportunity Q_(i−1) and T₃₁. Forexample, when the sum of backoff window sizes of 0th to (i−1)thretransmissions is 384, the (i−1)th ranging opportunity Q_(i−1)corresponds to 370, and delay is greater than 14 frames, a random accesstrial for the (i−1)th retransmission and a random access trial for thei-th retransmission may collide with each other. Accordingly, overlapoccurrence can be avoided by setting the i-th backoff window start pointto a maximum value between the sum of backoff window sizes of 0th to(i−1)th retransmissions,

${\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}},$

and the sum of the (i−1)th ranging opportunity Qi−1 and T₃₁.

Therefore, the i-th opportunity Q_(i) is set as follows.

$\begin{matrix}{Q_{i} = {{\max \left( {{\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}},{Q_{i - 1} + T_{31}}} \right)} + {{random\_ selection}{\_ under}{\_ P}_{i}{\_ window}{\_ size}}}} & \left\lbrack {{Math}\mspace{14mu} {Figure}\mspace{14mu} 6} \right\rbrack\end{matrix}$

In math figure 7, P₀ denotes the initial backoff window size K,P_(i)denotes the i-th backoff window size, and

$\max \left( {{\sum\limits_{k = 0}^{k = {i - 1}}\; P_{k}},{Q_{i - 1} + T_{31}}} \right)$

can be regarded as the backoff window start point of the i-thretransmission.

The present invention proposes a fourth method which adaptively supportsthe BEB that is a conventional backoff mechanism, the first method, thesecond method and/or the third method of the present invention. When theBEB and the second method of the present invention coexist, a wirelesscommunication system can be implemented such that the method ofincreasing an exponential backoff window size with factor 2 for rangingretry and the method of reducing the backoff window size with a specificfactor for ranging retry can coexist according to a specific systemenvironment. The base station can transmit a signal indicating a backoffwindow size suitable for the system environment of each mobile station.The base station can control a backoff mechanism used by a mobilestation by including a ranging backoff window indicator in a pagingmessage, a registration request message or a registration responsemessage and transmitting the message including the ranging backoffwindow indicator to the mobile station. For example, the base stationcan signal the mobile station such that the mobile station uses anincreasing mechanism which increases the backoff window size by settingthe ranging backoff window indicator having 1 bit to 0b0 and signal themobile station such that the mobile station uses a decreasing mechanismwhich decreases the backoff window size by setting the ranging backoffwindow indicator to 0b1. The base station expects that the mobilestation will transmit a ranging signal according to the above-mentionedmechanisms. Accordingly, the base station receives a ranging signaltransmitted by the mobile station according to an embodiment of thepresent invention within the backoff window according to an embodimentof the present invention.

While one backoff mechanism is applied to H2H devices, there are variousapplication types for M2M devices and the M2M devices can operate ingroups. The ranging backoff window size can be set using the decreasingmechanism for a group of M2M devices such as smart meters and theconventional BEB mechanism is applied to an M2M device group having asmall number of devices to perform random access with an increasedbackoff window size. By adaptively setting a random access scheme inthis manner, it is possible to reduce a potential collision count,decrease latency and provide a random access backoff method. The mobilestation and the base station can negotiate ranging backoff mechanismsapplicable to M2M devices through an AAI-REG-REQ/RSP to set a backoffwindow size. The base station can determine a ranging backoff mechanismapplied to each M2M device for each paging group. Both the increasingmechanism and the decreasing mechanism can be employed as the rangingbackoff mechanism according to characteristics of the mobile station (orM2M device). According to the present invention, the base station caninclude a ranging backoff window indicator which indicates the rangingbackoff mechanism applicable to the environment of the mobile station ina paging message and transmit the paging message to the mobile station.Furthermore, the mobile station and the base station can negotiatewhether the mobile station can support the ranging backoff decreasingmechanism through a registration request message or a registrationresponse message including the ranging backoff window indicator.

When the number of mobile stations is small, the mobile stations have touse the conventional BEB mechanism when requested to use ranging fordifferent purposes. That is, each mobile station can align the backoffwindow size according to BEB and use the system during a bandwidthrequest ranging procedure for requesting an uplink band when traffic tobe transmitted from the mobile station to the base station is generated.Therefore, it is possible to implement a random access scheme adapted tothe system.

The base station can transmit the ranging backoff window indicator overa channel which signals the initial backoff window size, or include theranging backoff window indicator in a paging message (AAI-PAG-ADVmessage in a 802.6m system) and transmit the paging message includingthe ranging backoff window indicator.

A ranging backoff window indication field may be adaptively applied inone system and implemented by a desired method of an operator.

FIG. 7 is a flowchart illustrating a random access backoff methodaccording to an embodiment of the present invention. A base stationtransmits a paging message including an initial backoff window size to amobile station (S701). The mobile station performs a ranging procedureon the basis of the initial backoff window size (S702). The mobilestation retransmits a ranging request message to the base station on thebasis of a second backoff window size when the mobile station does notsuccessfully receive a ranging acknowledgement message from the basestation (S703). When the mobile station fails to receive the rangingacknowledgement message from the base station again, the mobile stationretransmits the ranging request message to the base station on the basisof a third backoff window size (S704). When the ranging request messageaccording to a random access is successfully transmitted, the basestation transmits a ranging response message to the mobile station(S705).

The embodiments of the present invention can be applied to the mobilestation 100 and the base station 150 illustrated in FIG. 1. The mobilestation 100 of FIG. 1 may be an H2H device or an M2M device. Referringto FIG. 1, the processor 120 of the mobile station 100 controls thereceiver 112 to receive a message including backoff window informationfrom the base station 150. The processor 120 of the mobile station 100is configured to perform a ranging procedure on the basis of the backoffwindow information. The backoff window information includes informationindicating a 0-th backoff window size K₀. The processor 120 isconfigured to determine the x-th backoff window size K_(x) according toK_(x)=K₀/(2^(x)) where x denotes a ranging retry count. The processor120 can control the transmitter 111 to retransmit a ranging requestmessage on the basis of the backoff window information when the mobilestation 100 does not receive a ranging response message. In addition,the processor 120 can control the receiver 110 to receive the rangingresponse message in response to the ranging request message to the basestation 150, and perform a network entry procedure to the base station150 on the basis of the ranging response message. The x-th backoffwindow size can have a value greater than 2. The message can include aranging backoff window indicator which indicates an increase or decreasein the backoff window size for ranging retransmission. The processor 120can increase the backoff window size for ranging retransmission when theranging backoff window indicator indicates a backoff window sizeincrease and reduce the backoff window size for ranging retransmissionwhen the ranging backoff window indicator indicates a backoff windowsize decrease. Furthermore, the processor 120 can control the receiver112 to receive a message including the backoff window information fromthe base station 150. The processor 120 is configured to determine thex-th backoff window for x-th (x being a non-negative integer) rangingtransmission using the backoff window information. Moreover, theprocessor 120 controls the transmitter 111 to transmit the x-th rangingsignal to the base station 150 within the x-th backoff window and isconfigured to determine the (x+1)th backoff window start point inconsideration of the x-th backoff window end point.

Alternatively, the processor 120 can be configured to set an (x+1)thbackoff window start point to a larger value from the sum of the 0-thbackoff window size to the x-th backoff window size, and the sum of thex-th ranging transmission time and a predetermined time.

The processor 170 of the base station 150 can control the transmitter161 of the base station 150 to transmit a paging message includingbackoff window information to the mobile station 100 and be configuredto perform a ranging procedure on the basis of the backoff windowinformation. The processor 170 of the base station 150 can control thereceiver 162 to receive the ranging request message from the mobilestation 100. In addition, the processor 170 can control the transmitter161 to transmit a ranging response message in response to the rangingrequest message to the mobile station 100.

The processor 170 can control the receiver 162 to receive the x-thranging transmission from the mobile station 100 within an x-th backoffwindow determined using the backoff window information and control thereceiver 162 to receive the (x+1)-th ranging transmission from themobile station 100 within an (x+1)th backoff window determined using thebackoff window information. Furthermore, the processor 170 can beconfigured to determine the start point of the (x+1)th backoff window inconsideration of the end point of the x-th backoff window.

The embodiments of the present invention described herein below arecombinations of elements and features of the present invention. Theelements or features may be considered selective unless otherwisementioned. Each element or feature may be practiced without beingcombined with other elements or features. Further, an embodiment of thepresent invention may be constructed by combining parts of the elementsand/or features. Operation orders described in embodiments of thepresent invention may be rearranged. Some constructions of any oneembodiment may be included in another embodiment and may be replacedwith corresponding constructions of another embodiment. It is obvious tothose skilled in the art that claims that are not explicitly cited ineach other in the appended claims may be presented in combination as anembodiment of the present invention or included as a new claim bysubsequent amendment after the application is filed.

Those skilled in the art will appreciate that the present invention maybe carried out in other specific ways than those set forth hereinwithout departing from the spirit and essential characteristics of thepresent invention. The above embodiments are therefore to be construedin all aspects as illustrative and not restrictive. The scope of theinvention should be determined by the appended claims and their legalequivalents, not by the above description, and all changes coming withinthe meaning and equivalency range of the appended claims are intended tobe embraced therein.

INDUSTRIAL APPLICABILITY

While the present invention has been described and illustrated hereinwith reference to the preferred embodiments thereof, it will be apparentto those skilled in the art that various modifications and variationscan be made therein without departing from the spirit and scope of theinvention. Thus, it is intended that the present invention covers themodifications and variations of this invention that come within thescope of the appended claims and their equivalents.

1. A method for ranging transmission at a mobile station, the methodcomprising: receiving backoff window information indicating an initialbackoff size K₀ from a base station; determining an x^(th) backoffwindow size K_(x) according to K_(x)=K₀/(2^(x)), wherein the x is anumber of ranging retransmissions; and performing the rangingtransmission within the backoff window size K_(x).
 2. The methodaccording to claim 1, wherein the backoff window information is includedin a paging message.
 3. The ranging transmission method according toclaim 1, wherein the initial backoff size K₀ is greater than
 2. 4-6.(canceled)
 7. A mobile station configured to perform rangingtransmission to a base station in a wireless communication system, themobile station comprising: a receiver; a transmitter; and a processorconfigured to control the receiver, wherein the processor is configuredto control the receiver to receive backoff window information indicatingan initial backoff size K₀ from the base station and to determine anx^(th) backoff window size K_(x) according to K_(x)=K₀/(2^(x)), whereinthe x is a number of ranging retransmissions; and to perform the rangingtransmission within the backoff window size K_(x).
 8. The mobile stationaccording to claim 7, wherein the backoff window information is includedin a paging message.
 9. The mobile station according to claim 7, whereinthe initial backoff size K₀ is greater than
 2. 10-12. (canceled)